1. Field of the Invention
The present invention relates to multilayer inductors, and in particular, the present invention relates to a surface-mount type multilayer inductor used in choke coils, LC filters, transformers, and balun (balanced-to-unbalanced) transformers.
2. Description of the Related Art
A conventional coil element is disclosed in Japanese Unexamined Patent Application Publication No. 5-41324. The coil element is provided with a column-shaped magnetic core including a magnetic body such as a ferrite having an insulation resistance. On the surface of the magnetic core, a conductive film is provided, and the conductive film is irradiated with a laser beam and moved in the axial direction while being rotated, such that a spiral groove defining a coil is formed, and a coil spirally wrapped around the magnetic core is formed from remnants of the conductive film. The above publication also discloses that two or more coils may be provided by cutting the conductive film.
In conventional coil elements, there are several ways to obtain a large value of inductance including: using a magnetic core having a large cross-section, increasing the number of turns of the coil, and forming the magnetic core from a material having a large value of permeability xcexc. However, the magnetic core is naturally limited in permeability xcexc and in size (cross-sectional area, length), so that a desired inductance value has been difficult to obtain. It is virtually impossible to achieve a desired inductance value especially when forming plurality of coils on one magnetic core.
To overcome the above-described problems with the prior art, preferred embodiments of the present invention provide a small-sized multilayer inductor having a large inductance value.
A multilayer inductor according to a preferred embodiment of the present invention includes a core member, a plurality of thin-film coils provided on the surface of the core member with an insulation layer interposed therebetween, the thin-film coils being wound around the outer periphery of the core member, and terminal electrodes arranged at ends of the core member and electrically connected to each end of the thin-film coils, wherein each of the terminal electrodes is defined by providing a dividing groove on the core member from an end surface thereof to the outer periphery thereof such that the terminal electrode is electrically insulated from the other terminal electrode. Each of the thin-film coils is preferably providing by forming a spiral coil-forming groove on a thin-film conductor provided on the outer periphery of the core member.
The core member may be drum-shaped, for example, including a distinguishing portion provided on at least one of the end surfaces of the core member and a side thereof for distinguishing the orientation of the core member. Furthermore, preferably, the starting and finishing ends of at least one of the thin-film coils are electrically connected to the respective terminal electrodes via connection openings provided in the insulating layer.
With the structure described above, the length of the core member is greatly reduced and the number of turns of the thin-film coils is greatly increased in comparison with an inductor in which two thin-film coils are arranged in a row in the axial direction of a core member. Also, a plurality of thin-film coils are coaxially arranged on the core member with the insulation layer interposed therebetween, such that the distributed capacity between the thin-film coils is substantially uniform. Furthermore, the starting end and the finishing end of each coil-forming groove are shifted with each other by substantially 180xc2x0 in the winding direction of the core member, and the starting ends of two adjacent thin-film coils having the insulation layer interposed therebetween are shifted with respect to each other by substantially 180xc2x0 in the winding direction of the core member, such that each terminal electrode has substantially the same shape and substantially the same area as each other.
The thin-film coil winds spirally around the outer periphery of the core member at a predetermined pitch. Therefore, the length of the dividing groove between a point intersecting with a spiral coil-forming groove and an end surface of the core member is greater than the lengths of the dividing groove between the starting end of the spiral coil-forming groove and each end surface of the core member and between the finishing end of the spiral coil-forming groove and each end surface of the core member, such that the terminal electrodes are securely and electrically insulated from each other.
When a dividing groove winding around from the end surface of the core member toward the outer peripheral surface thereof is provided, similar advantages can be achieved. In this case, the starting and finishing ends of the spiral coil-forming groove for each thin-film coil are respectively positioned on a single plane of the core member, so that each terminal electrode may have substantially the same shape and substantially the same area as each other.
Other features, elements, advantages and characteristics of the present invention will become more apparent from the detailed description of preferred embodiments thereof with reference to the attached drawings.